Climate change from human activity nearly doubled the area that burned in forest fires in the American West over the past 30 years, a major new scientific study has found, and larger, more intense fires are all but guaranteed in the years ahead.

On public and privately owned forest lands, 23.5 million acres burned in the 11 Western states from 1984 to 2015. Climate change was responsible for roughly 10.4 million of those acres — an area 30 times the size of the city of Los Angeles — because of hotter and drier conditions than otherwise would have occurred, the study from scientists at Columbia University and the University of Idaho found.

The increase is coming at a large cost, not only in lives and property but also in taxpayer expense. Last year, the federal government spent $2.1 billion fighting fires.

“No matter how hard we try, the fires are going to keep getting bigger, and the reason is really clear,” said Park Williams, a climate scientist at Columbia’s Lamont–Doherty Earth Observatory in New York and co-author of the research. “Climate is really running the show in terms of what burns.”

Driven by the burning of coal, gasoline, oil and other fossil fuels that trap heat in the Earth’s atmosphere, the 10 hottest years globally since modern records began in 1880 all have occurred since 1998, according to NASA. Last year was the hottest year in that 135-year record. The previous hottest was 2014. And 2016 is on track to break the record again.

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Across the vast forests of the American West, the number of fires, the size of the area burned and the length of the fire season all have been increasing in recent decades.

“There’s a difference from when I started my career 20 years ago,” said Scott McLean, a spokesman for the California Department of Forestry and Fire Protection, known as Cal Fire. “Fires are burning hotter and quicker, and more severe, more intense. To us, climate change is a proven fact.”

The states included in the study were California, Oregon, Washington, Idaho, Utah, Nevada, Arizona, New Mexico, Montana, Colorado and Wyoming. During the 10 years from 1984 to 1993, a total of 3.4 million acres of forest burned there, the researchers found. But 20 years later, from 2004 to 2013, there were 10.4 million acres of forests that burned. The study was published Monday in the Proceedings of the National Academy of Sciences, one of the nation’s leading scientific journals.

For Californians, the impact of the damage is close to home. The Soberanes fire, which began July 22 and has burned more than 132,000 acres in Big Sur, has become the most expensive wildfire in U.S. history. It surpassed $236 million last week for the equipment, food and wages of fire crews and other related costs over 80 days. The blaze is now 99 percent contained, but at its peak, more than 5,000 firefighters from around the United States battled the Soberanes fire, which began with an untended campfire in Garrapata State Park, then destroyed 57 homes and killed a bulldozer operator.

State fire commanders have been bringing on more firefighters, and keeping them on duty for longer stretches, than ever before, said McLean.

“We’ve seen an increase in our fire season of roughly 70 days a year over the past 50 years,” he said. “Look at Southern California. The fire season there is year-round now.”

To be sure, climate change is not the only reason for the increase in Western forest fires. A century of fire suppression, which has built up unnaturally dense and often unhealthy pine and fir forests combined with more people building homes in rural areas, is also partially to blame, the scientists concluded.

But simply put, the West is hotter and drier now than it was a generation or two ago. Since 1970, the average temperature in forested parts of the West has increased by 2.5 degrees Fahrenheit. Soils are more arid, and trees and brush have less moisture. One spark has a higher chance of doing immense damage.

Looking at eight measurements of weather patterns and moisture levels, including the Palmer Drought Severity Index and the MacArthur Forest Fire Danger Index, the scientists in Monday’s study measured “fuel aridity” of the trees and brush in forests. They compared acres burned and found a close link between aridity and fire. When they crunched the totals with supercomputer climate change models, they concluded that 55 percent of the increase in fuel aridity in the past 30 years can be attributed to climate change.

Centuries ago, lightning and Indians clearing land burned more acres a year than are burning now in California, said Scott Stephens, a fire scientist at UC Berkeley who did not participate in the study. But those fires were mostly low-intensity affairs, helping clear dead underbrush. Today, because fire crews have put out blazes for generations, many forests have so much dead and living vegetation that they often explode out of control, wiping out large trees and seeds, Stephens said.

The solution, he said, is more controlled burns during wet months and logging to thin forests to restore their natural condition. Both can be expensive and politically unpopular.

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“My back-of-the-envelope calculation says we need to do 10 times the area we are doing today annually to make a difference,” he said. “This is very challenging but necessary.”

Williams said higher temperatures and overgrown forests will mean larger fires for decades. After areas have burned, he said, some will grow back as before, like the lodge pole pine forests of the Northern Rockies, while others, like in parts of California, may shift from conifers to more chaparral, manzanita and oak. Most ominous, in the past 30 years only about 10 percent of forested areas in the West have burned.

“It means getting out of fire’s way,” he said. “I’d definitely be worried about living in a forested area with only one road in and one road out.”

Paul Rogers has covered a wide range of issues for The Mercury News since 1989, including water, oceans, energy, logging, parks, endangered species, toxics and climate change. He also works as managing editor of the Science team at KQED, the PBS and NPR station in San Francisco, and has taught science writing at UC Berkeley and UC Santa Cruz.

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